The present invention relates to a method of producing an ultrafine particle predispersion. More particularly, the present invention relates to an ultrafine particle dispersion producing method capable of producing an ultrafine particle dispersion for polishing surfaces of semiconductor devices, magnetic recording media and so forth without using a dispersant and without mixing a contaminant or the like into the dispersion.
Fine particles are used in various fields. For example, fine particles used in burning in the field of electronic industry are required to improve in purity, density, etc. For example, fine particles of barium titanate, which is a material used for a ceramic capacitor and the like, indispensably need to increase in purity to a high level or improve in density in order to attain an improvement in performance of the capacitor by stabilization of the burned configuration thereof.
Meanwhile, methods of producing semiconductor devices or magnetic recording media include precisely polishing surfaces to obtain mirror-finished surfaces. In these precision polishing methods, a fine particle dispersion for polishing is used. In particular, semiconductor devices are now highly integrated and also increased in the number of layers constituting a multilayered structure. Semiconductor devices having a multilayered structure need to planarize the surface of an interlayer insulator film formed between a pair of adjacent layers. In the semiconductor manufacturing process, the interlayer insulator film is planarized by polishing. Meanwhile, metal wiring is formed by a vacuum deposition means. In this case, it is necessary to smooth the deposited wiring film by removing minute unevenness varying in size and density.
In the process of polishing an oxide film formed of silicon dioxide to obtain a flat surface, colloidal silica with potassium hydroxide added thereto is used. In polishing a metal wiring film, the metal is polished chemically and mechanically by using a slurry prepared by mixing together an abrasive and an oxidizing agent.
Fine particle dispersions used in the chemical/mechanical polishing method are produced by dispersing particles of silica, alumina, zirconia, titania, ceria, manganese oxide, iron oxide, etc. using any of medium type dispersers, e.g. a bead mill, a ball mill, and a sand mill, stirring type dispersers, e.g. a colloid mill, and an ultrasonic disperser.
Fine particle dispersions used for chemical/mechanical polishing contain fine particles having a primary particle size of from 10 to 100 nanometers. A fine particle powder is dispersed in an alkaline or acidic aqueous solution. At the same time as the powder is dispersed, an electrical double layer is formed near the surfaces of the fine particles by ions in the aqueous solution, and the xcex6-potential of the particles in the slurry reduces. Accordingly, the attractive forces between the particles increase, causing an agglomeration phenomenon to occur. Consequently, the particles reach a stable state in the form of agglomerates of the order of from 300 micrometers to 1 millimeter in size. In the present semiconductor manufacturing process, a particle size of 140 to 200 nanometers in terms of center particle size is demanded for fine particle dispersions used in chemical/mechanical polishing, and the breadth of the particle size distribution demanded is from 100 to 400 nanometers. Therefore, the agglomerates need to be redispersed by some method.
When only a stirring type disperser is used to redisperse the fine particles for polishing, most of the agglomerates cannot be redispersed even if the treatment is carried out for a considerably long period of time.
In the case of a fine particle dispersion for chemical/mechanical polishing that uses silica particles as an abrasive, an alkaline solution prepared by dissolving potassium hydroxide in ultrapure water is mixed with 13% to 25% by weight of silica having a primary particle size of from 20 to 30 nanometers to obtain a dispersion. The obtained dispersion is stirred at high speed for 1 hour at 3,000 rpm and further subjected to dispersing treatment for 1 hour at 1,400 rpm in a bead mill using beads having a diameter of 2 millimeters, thereby obtaining a fine particle dispersion for polishing that has a center particle size of 230 nanometers and a viscosity of 6 to 10 mPaxc2x7s.
In the case of using a medium type disperser, e.g. a ball mill, a fine particle dispersion having a center particle size of the order of 200 nanometers and a particle size distribution breadth of from 150 to 700 nanometers is obtained. Thus, it is difficult to obtain a sharp particle size distribution. In addition, as the period of time of treatment becomes longer, the medium itself is worn by a larger amount, causing the fine particle dispersion to be contaminated. This may lead to contamination of semiconductor devices.
Fine particle dispersions obtained by the above-described dispersing method differ in treating characteristics for each batch of agent owing to the change with time. Consequently, it is impossible to provide consistency in polishing results. Moreover, there is a problem that the slurry settles in a tank for supplying an amount of abrasive slurry sufficient to polish a number of wafers for one day, which is known as a xe2x80x9cday tankxe2x80x9d; therefore, it is essential to discharge the slurry from the tank to the outside and dispose of it.
High-purity fine particles are also demanded in the fields of paper, cosmetics, paint, food and so forth. In the field of paper industry, for example, it is demanded that fine particles used as an inner filling material and surface modifying material should be purified to a higher level. In addition, the use of a highly concentrated fine particle dispersion is demanded in order to obtain high paper quality.
In fine particle dispersions used as food additives, it is also demanded to improve absorptivity by atomization and to obtain contamination-free fine particles.
However, it is extremely difficult to obtain a fine particle dispersion in which fine particles are stable in a suspension state. Accordingly, a method of obtaining a high-purity fine particle dispersion within a short period of time without using particles or the like for dispersion has been demanded.
When a fine particle dispersion of a single composition or a plurality of compositions is suspended in a suspending medium suitable therefor, fine particles often float on the surface of the suspending medium without sinking into it in the case of an ordinary introducing and stirring method because fine particles have a large surface area and hence an extremely small bulk specific gravity in comparison to the true specific gravity.
When a fine particle dispersion of a single composition or a plurality of compositions is suspended in either a suspending medium unsuitable therefor, or when a fine particle dispersion of a plurality of compositions of different nature is suspended in either of the suspending mediums, even if the fine particles are introduced into the suspending medium, a considerably long time is required to obtain a uniform predispersion in the case of preliminary stirring by a stirring machine or the like. Consequently, the fine particles undesirably agglomerate before it is sufficiently stirred in the suspending medium. Thus, it is difficult to obtain a uniform predispersion.
Under these circumstances, Japanese Patent Application Unexamined Publication (KOKAI) Nos. 10-310415 and 11-57521 disclose a method in which fine particles are predispersed in water while being sucked by a powder introducing and mixing disperser (trade name: Jet Stream Mixer). According to the method, fine particles are introduced directly into a dispersing medium and stirred at the same time to perform preliminary dispersion. In this method, blades for stirring are rotated at high speed to produce a negative pressure to introduce fine particles together with air. Such a system uses an apparatus in which a lubricating oil is used in the rotating shaft part to allow the stirring blades to stably rotate at high speed.
FIG. 12 is a diagram illustrating a conventional suction stirring apparatus.
A suction stirring apparatus 71 is installed in a suspension tank 72. A rotor 74 connected to a rotating shaft 73 is rotated at high speed by a motor 75. As a result, a negative pressure is formed in the vicinity of the rotor in a suspending medium 76, causing fine particles 78 in a fine particle storage tank 77 to be sucked through a suction flow path 79, which is formed around the rotating shaft 73, and injected into the suspending medium 76 in the suspension tank 72. In addition, a cylindrical stator 80 is formed around the rotor 74 to induce a circulating flow 81 circulating through the inside and outside of the cylindrical stator 80. Thus, mixed dispersing is performed by the circulating flow 81 and the shearing force of the rotor 74.
In order to ensure the suction flow path 79 to suck fine particles together with air, the apparatus needs to prevent air from flowing thereinto through any portion except a duct 82 for introducing fine particles. Accordingly, the upper portion of the rotating shaft has a hermetically sealed structure. A hermetic structure using a mechanical seal is used for a bearing portion 83 that rotates at high speed, i.e. from 3,000 to 4,000 RPM.
As a mechanical seal used in such a bearing portion, it is essential to use an oil-filled seal because the bearing portion is a part that rotates at high speed. An oil-filled mechanical seal cannot prevent the oil filled therein from penetrating along the rotating shaft by the action of a negative pressure produced in the pipe as the rotating shaft rotates. Therefore, it is difficult to completely prevent oil from adhering to fine particles when the particles pass through the suction flow path formed around the rotating shaft. Thus, the oil-filled mechanical seal suffers from the problem that a high-purity predispersion cannot be obtained.
Moreover, when fine particles are sucked, air is also introduced together with the fine particles. Consequently, air undesirably remains as fine bubbles in the suspension. Therefore, if it is intended to disperse the predispersion through collision under pressure without using particles for dispersion, the bubbles in the suspension undesirably act as a buffer, reducing the pressurizing efficiency, and thus making it difficult to disperse the predispersion satisfactorily.
If it is intended to disperse the predispersed suspension to a high degree by a dispersing apparatus using particles for dispersion, e.g. a bead mill, a ball mill, or a sand mill, an extremely long time is required. Consequently, contaminants are generated from the particles for dispersion, causing the purity to be reduced. Moreover, it is difficult to obtain a dispersion in which fine particles are uniformly dispersed.
Accordingly, methods of producing a dispersion without using particles for dispersion are proposed, for example, in Japanese Patent Application Unexamined Publication (KOKAI) Nos. 9-193004, 9-142827, 10-310415 and 11-57521. Dispersing methods employed in these methods use dispersing apparatus based on a high-pressure impact system, an opposed impact system, etc. The first system is arranged to obtain a fine particle dispersion by jetting out a predispersed fine particle suspension from a nozzle under high pressure so that the fine particle suspension collides against a plate having a high hardness (trade name, Manton-Gaulin Homogenizer: Doei Shoji). With this system, however, the plate member wears out at a high rate, and consequently, the problem of contamination cannot be solved. The second system is arranged to obtain a fine particle dispersion as follows. After a high pressure has been applied to a predispersed fine particle suspension, the flow path is branched, and after the fine particle suspension has been once made to collide against a plate member, the branched flow paths are changed in direction through 90 degrees so that fine particle dispersions collide with each other (trade name, Microfluidizer: Mizuho Kogyo; Nanomizer: Tsukishima Kikai; Genus PY: Hakusui Kagaku Kogyo, etc.). However, in the process of changing the flow path through 90 degrees by collision after branching the flow path under high pressure, impact caused by the collision against the plate member is large. Even when diamond is used, durability is very low. Moreover, the increase in throughput capacity is unfavorably limited by a structural problem.
According to a third system, after a high pressure has been applied to a predispersed fine particle suspension, the flow path is branched into two, and fine particle suspensions are made to collide with each other directly by nozzles opposed to each other, thereby obtaining a fine particle dispersion (trade name, Ultimizer: Karasawa Fine). The third system is superior to the first and second systems. However, it is practically impossible to oppose two nozzles to each other at a predetermined distance so that the nozzles are completely parallel to each other and the center lines of the nozzles are completely coincident with each other. Thus, there have been problems to be solved in terms of the generation of contaminants, mass productivity, durability, etc.
An object of the present invention is to provide a method of producing a high-purity fine particle dispersion without using a particulate substance for dispersion, the method being capable of producing contaminant-free high-purity fine particles within a reduced period of time. Another object of the present invention is to provide a method of producing a fine particle dispersion of favorable dispersion stability that will not thicken to gel or form a precipitate even if it is stored for a long period of time.
The present invention is a method of producing a fine particle dispersion characterized by having a dispersing step where, after fine particles have been sucked into a dispersing medium to prepare a suspension by a suction type stirring machine and bubbles have been removed from the suspension by a bubble removing means, the suspension is pressurized and introduced from opposite directions so as to collide with each other, thereby dispersing the suspension.
In the above-described method of producing a fine particle dispersion, the suction-stirring machine is arranged such that only a flow path for an air flow is formed in a space where a rotating shaft is exposed, and a flow path for fine particles is formed outside the flow path for an air flow.
In the above-described method of producing a fine particle dispersion, the bubble removing means is a cyclone type bubble removing means.
In the above-described method of producing a fine particle dispersion, a dispersing means capable of adjusting a center axis of a dispersing nozzle is used as one of two dispersing nozzles in the dispersing step.
In the above-described method of producing a fine particle dispersion, a dispersing means having a dispersing nozzle in which the cross-sectional area gradually increases from the inlet side thereof toward the outlet side thereof is used in the dispersing step.